Electric power conversion systems – Current conversion – Having plural converters for single conversion
Reexamination Certificate
2001-12-04
2003-10-21
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
C363S132000, C323S259000
Reexamination Certificate
active
06636431
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Technical Field
This invention relates to a DC/DC converter for carrying out a step-up or a step-down operation upon transferring an electric energy and, in particular, to a symmetrical DC/DC converter.
(2) Background Art
For example, an electric double layer capacitor is known as an energy storage device having a voltage variable in dependence upon the amount of energy stored therein. When the electric double layer capacitor is used, a step-up or a step-down operation by a DC/DC converter is required when energy transfer is carried out upon charging or discharging.
In case where a motor of an electric automobile is used as a load to consume an electric energy of the electric double layer capacitor, the motor may conversely generate an electric energy, for example, upon braking to act as a power generator. If the electric energy produced by the motor is recovered, it is possible to considerably improve an energy efficiency of a whole system including the electric double layer capacitor and the motor as a power supply and a load, respectively.
Therefore, it is important that the DC/DC converter for carrying out the step-up or the step-down operation has a function of transferring the electric energy in one direction from one terminal portion to the other terminal portion and in the other direction from the other terminal portion to the one terminal portion, whichever terminal portion acts as an input terminal portion to be supplied with the electric energy. To this end, it is proposed to provide a pair of DC/DC converters for the respective directions and a switch for selecting one of the DC/DC converters. However, this structure is not practical because the size of the whole system is increased. In view of the above, it is indispensable to provide a DC/DC converter which, as a single circuit, is operable even if an input terminal portion and an output terminal portion are exchanged.
As the DC/DC converter of the type described, proposal has already been made of a so-called bidirectional converter.
A first existing bidirectional DC/DC converter is a circuit device connected between a battery or a power supply (such as an electric double layer capacitor) and a load (such as a motor) and has a plurality of switching circuits and four terminals.
The first existing bidirectional DC/DC converter is operable in the following manner. In case where the electric double layer capacitor supplies the electric energy to the motor to drive the motor, one of the switching circuits is put into an opened state while the other switching circuit is operated under PWM (Pulse Width Modulation) control or the like. In this event, the DC/DC converter is operated as a forward converter circuit. On the other hand, in case where the electric energy generated by the motor is supplied to the electric double layer capacitor to charge the electric double layer capacitor, the other switching circuit is put into an opened state while the one switching circuit is operated under the PWM control or the like. In this case, the DC/DC converter is operated as the forward converter circuit.
Japanese Unexamined Patent Publication No. 2000-333445 (JP 2000-333445 A) discloses a second existing bidirectional DC/DC converter connected between a d.c. power supply and a load including a capacitor. The second existing bidirectional DC/DC converter uses FETs (Field Effect Transistors) as switching devices. Specifically, the second existing bidirectional DC/DC converter comprises a first series circuit, a second series circuit, an inductor, and a control unit. The first series circuit comprises first and second FETs connected in parallel to the d.c. power supply. The second series circuit comprises third and fourth FETs connected in parallel to the load. The inductor is connected between a junction of the first and the second FETs and another junction of the third and the fourth FETs. The control unit controls respective gates of the first, the second, the third, and the fourth FETs so that the electric energy is supplied from the d.c. power supply to the load and that the electric energy stored in the capacitor is recovered and fed back to the d.c. power supply.
The above-mentioned first existing bidirectional capacitor comprises a plurality of inductors and has a complicated circuit structure.
In case where the energy storage device, such as the electric double layer capacitor, having a variable voltage is used or in case where the motor is driven as the load and conversely produces the electric energy to be recovered, the system includes voltage variation corresponding to the amount of energy. Therefore, it is necessary not only to transfer the energy in two directions but also to desiredly carry out the step-up or the step-down operation in correspondence to the status of energy at the electric double layer capacitor (power supply) and the motor (load).
In case where the system is constructed by a combination of a plurality of energy storage devices, a plurality of power generators, and a plurality of loads, it is necessary to set an appropriate step-up ratio or an appropriate step-down ratio of the DC/DC converter connected to these components. Thus, each of the existing bidirectional DC/DC converters is not applicable to such system.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a symmetrical DC/DC converter which is operable in a variable direction corresponding to an energy transfer direction at a step-up or a step-down ratio corresponding to an input/output voltage ratio.
According to this invention, there is provided a symmetrical DC/DC converter comprising a single inductor with a pair of switching means connected to its terminals in a symmetrical arrangement with respect to the inductor, the converter being operable as a step-up (boost) converter and a step-down (buck) converter in a manner such that one and the other of the switching means are used as an input switch and an output switch, respectively, and that one and the other of the switching means are conversely used as an output switch and an input switch, respectively.
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Patent Abstract of Japan, vol. 2000, No. 14, Mar. 5, 2001 and JP 2000-333445 A (Toyota Corp.) Nov. 30, 2000—Abstract only.
F. Caricchi et al: “20 kW Water-Cooled Prototype of a Buck-Boost Bidirectional DC-DC Converter Topology for Electrical Vehicle Motor Drives”, IEEE, pp. 887-892, XP01014767—entire document. No Date.
F. Caricchi et al: “Study of Bidirectional Buck-Boost Converter Topologies for Application in Electrical Vehicle Motor Drives”, IEEE, pp. 287-293, XP101263608—Figure 2. No Date.
“On the Buses”, New Electronics, Apr. 18, 2000, pp. 35-36, XP001011749—Figure 1.
Ito Takahiko
Seki Sumiko
Frishauf Holtz Goodman & Chick P.C.
NEC Tokin Corporation
Patel Rajnikant B.
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